Recently, with the rapid development of the industry related to portable electronic devices such as smart phones, there is a substantially increasing demand for lithium-ion batteries and lithium polymer batteries, which are representative secondary batteries. In particular, facing an age of high oil price, accompanying practical use of electric vehicles such as hybrid vehicles and plug-in vehicles, it can be expected that the demand for lithium secondary batteries will explosively increase in the future. Along with such industrial demand, there is a demand for reduction in weight and size and increase in capacity of lithium secondary batteries as new technical challenges.
Separators, which are main components that influence the performance of the secondary battery are inserted between anodes and cathodes, and function to prevent a short-circuit phenomenon in which the cathodes and the anodes are brought into contact with each other. Also, in each separator, infinitely numerous micro pores are formed and through the pores, ionic substances are transferred between the anode and the cathode and thereby charged and discharged repeatedly.
Such separators each mainly includes a polyolefin resin that is excellent in chemical stability and electrical characteristics, and the separators are different in performance, that is, mechanical strength and electrical performance, depending on the size, distribution proportion and orientation structure of the pores that serve as passages of ionic substances. Accordingly, various techniques for pore structures of separators have conventionally been developed.
For example, Korean Patent No. 373204 (registration date: Feb. 10, 2003) describes a multicomponent composite separator for a polyelectrolyte configured so as to include an active layer at each of surfaces on opposite sides thereof and a support layer inside thereof, and have a Gurley value of no more than 20,000 seconds/100 cc.
Also, Korean Patent No. 577731 (registration date: May 1, 2006) describes a microporous separator for a secondary battery in which a polyolefin resin is used as a main material, an amorphous layer having a fixed thickness is formed on each of surfaces on opposite sides thereof, a crystal layer is formed inside thereof, sizes of pores of the amorphous layer are no more than 1 μm, and sizes of pores of the inside crystal layer are no more than 5 μm and the entire porosity is no less than 50%.
Also, Korean Patent No. 776029 (registration date: Nov. 6, 2007) describes a polyolefin separator for a secondary battery in which at a cathode-side surface thereof, the distribution proportion of pores of sizes of 80 nm to 2 μm is 90 to 97% and the distribution proportion of pores of sizes of less than 80 nm is 3 to 10% and at an anode-side surface thereof, the distribution proportion of pores of sizes of 30 nm to 1 μm is 90 to 97% and the distribution proportion of pores of sizes of less than 30 nm is 3 to 10%.